Electrochemical tools to measure local cerebral blood flow and metabolism The human brain comprises 2% of body weight and yet consumes 20% of the total energy to support the electrical activity in the brain. To meet this demand, cerebral blood flow (CBF) is coupled to changes in energy use. There are two primary control points. Blood vessels near the brain surface maintain blood pressure in a relatively constant range according to the physiological requirements. CBF is also locally regulated within brain tissue at the level of the intraparenchymal microvessels, the subject of the proposed research. Microvessels spaced approximately 50 5m apart within brain tissue, are associated with smooth muscles that allow them to constrict and dilate, a process termed functional hyperemia. The chemical signaling that controls the capillaries' diameter primarily originates from astrocytes, glial cells that surround the blood vessels and also surround synapses. Neural activity activates astrocytes, which, in turn, secrete substances that trigger local vasodilation of capillaries. Here, we propose to test the role of serotonin (5-HT) on CBF in brain regions where it is present in high amounts. 5-HT is a potent vasoconstrictor, and its terminals are adjacent to arterioles and capillaries suggesting it can exert action directly on blood vessels or indirectly by activation of astrocytes. These actions are of importance because they have been suggested to be involved in migraine headaches and sudden infant death syndrome, and also to contribute to the signals observed in functional magnetic resonance imaging. My laboratory has developed several electrochemically based techniques for in vivo measurements of neurotransmitters as well as species governed by CBF such as O2 and pH. This proposal has 4 specific aims. 1. We will microfabricate an electrochemical probe of CBF based on the hydrogen clearance technique. This is necessary because the concentration of extracellular O2 is equal to the amount delivered by CBF less the amount consumed by ongoing metabolism. While extracellular O2 is readily measured with microelectrodes, to fully characterize functional hyperemia, a method to measure CBF is required. 2. We will test the hypothesis that 5-HT plays a unique role in the control of blood flow in select brain regions. Our primary target will be the substantia nigra, pars reticulata, a brain region with high 5-HT. 3. We will test the hypothesis that extracellular O2 in the nucleus accumbens core (NAc) fluctuates during reward based activity. Measurements will be made in the freely moving rats during intracranial self-stimulation while monitoring oxygen and biogenic amine neurotransmitters. 4. We will test the hypothesis that extracellular O2 changes in the substantia nigra during ICSS. This region contains the dendrites of dopamine neurons and is involved in the regulation of locomotor activity. We will make chemical measurements of O2 and 5-HT and electrophysiological measurements of GABAergic neurons in this region with the same electrode during ICSS.